The invention relates to a high-voltage supply means for power electron-beam guns with a heatable cathode and an anode, particularly for electron-gun vaporizers, with a high-voltage transformer, smoothing means for the residual waviness of the feed voltage, an electronic switching member, which is arranged in the current circuit to the cathode and a switching circuit which is controlled dependent on current and which cuts in to produce a blocking voltage for operating the switching member.
The expression "power electron-beam guns" is usually understood to mean such guns operating in a power range of between 2.0 and 300 kW. Depending upon gun capacity and application, high voltage supply means suitable for this purpose have an output voltage of between 4 and 150 kV. Because of the conditions affecting their use, such high-voltage supply means must be short-circuit proof, and this will be discussed in greater detail hereinafter.
Electron-beam guns of the stated kind are mainly operated in a vacuum, i.e. in a vacuum chamber, the vacuum being greater than 10.sup.-3 mbars. Under normal pressure and because of the inevitable collisions of the electrons with gas molecules, electron beams cannot travel distances appreciably greater than 20 to 25 mm. However, within the vacuum chambers in which the electron-beam guns are located, the above-mentioned collisions can cause ion formation which leads to voltage flash-over, i.e. to conditions resembling short-circuiting. The deposition of vapour on, and the contamination of, insulators and other voltage-carrying components, as promoted in a vacuum, can also lead to periodic short circuits in the high voltage system. A certain degree of resistance to short-circuiting can be achieved by switching off the high voltage as promptly as possible when a predetermined current-limit value is exceeded. After a likewise predetermined time, which is on the order of 20 to 1000 milliseconds, has elapsed, and which normally suffices for de-ionizing the discharge sections, voltage is restored by an automatic control means. Depending upon the construction of the high-voltage supply means, the switching-off operation can be carried out in various ways. This can be done, for example, by means of relays, electronic contactors or thyristors provided on the primary side of low-voltage side. On the high-voltage side it is possible to switch off by means of mercury tubes, or to limit the current with the aid of regulating tubes. The switching arrangements used for this purpose form part of the prior art and therefore do not need to be described in detail.
It has been found in practice that under certain unavoidable conditions, extremely "hard" short-circuits can occur in the current circuit in the vacuum chamber. The expression "hard" short-circuits will be understood as meaning such short circuits as are characterized by a steep rise in current. For example, short-circuits have been measured in which the current downstream of the high-voltage filter capacitor used as the smoothing means has risen, in less than five microseconds, to values which were limited only by the internal resistance of the mercury tubes and the resistance of the high-voltage cables. The internal resistance of the current source corresponds in this case to the extremely low internal resistance of the charged filter capacitor. Within less than five microseconds, such current assumed 50 to 100 times the value of the nominal voltage of the supply means.
The above-mentioned discharges have quite a number of disadvantageous effects. Because of the steep rise in current, which reaches its maximum before the switching member present can block the current, there even occurs, in the case of high-voltage supply means having switching members consisting of mercury tubes, ionization phenomena in these tubes that can lead to their failure. It is no longer possible to switch off the supply means, so that the safety devices respond and/or the mercury tubes are destroyed. On the other hand, the described discharges cease immediately if insufficient energy is supplied for maintaining the discharge. A steep rise in current and likewise a steep drop in current which occurs when the discharge ceases to take place can cause marked reversals of impulses in adjacent leads because or magnetic induction, which reversals can lead to the destruction of other electrical components.
The problems occur principally in the case of high-voltage supply means having switching or current-limiting tubes arranged on the high-voltage side; however, they also play a part in the case of high-voltage supply means wherein relays or thyristors are arranged on the low-voltage side for the purpose of switching off.